The method described in the present invention is for reducing the size of the steps in the solvent extraction process, whereby the reduction occurs by decreasing the size of the middle cells of the extraction step. The invention also focuses on the cells used in the solvent extraction step.
Liquid-liquid solvent extraction is often used as a part of the hydro-metallurgical production of metals. The solvent extraction process usually consists of several stages or steps, where for instance pre-extraction, extraction, re-extraction etc. take place. Each extraction step contains several extraction cells, in each of these generally has a pump unit, which transfers the liquid phases of the extraction from one cell to another; one or several mixing units i.e. a mixer; and a settling unit behind the mixer i.e. a settler, where the liquids dispersed in the mixer settle into their own phases by force of gravity. A pump may also be combined with a mixer, so that the mixer also functions as a pumping device. In the reactions that occur during the solvent extraction process, one or more metals move from one phase (generally the aqueous phase) to another (organic) phase and can then be separated using re-extraction. The function of the extraction cells is to provide good conditions for the progression of the extraction reactions and finally to separate the liquids from each other.
The construction of the solvent extraction process varies considerably depending on which metal is being extracted. Copper extractions are usually fairly simple in that they only consist of a few extraction and re-extraction steps, but on the other hand, the size of copper extraction cells has grown enormously with time, which imposes its own limits on the extraction. The extraction processes of other metals are, however, much more complicated as far as the quantity of steps is concerned, although their capacity is a fraction of the amount of copper produced in an extraction plant.
In the solvent extraction cells used at present, almost all the afore-mentioned components are present: a pump, one or several mixers, an optional pre-settler and the main settler. The phases being mixed are pumped from one cell to another, where they disperse, any mixing and reaction time required for transferring of metal ions from one phase to another occurs in the mixer and the pure phases are separated from each other in the settler. These methods are described for example in U.S. Pat. Nos. 5,185,081 and 5,662,871. Within the extraction steps, all the extraction cells are similar, i.e. the separating capacity of phases is optimal in all cells.
U.S. Pat. No. 4,391,711 describes a solvent extraction arrangement, comprising one chamber, into the center of which the dispersing phases are conducted under a mixer. The space surrounding the mixer forms a dispersion band, above which there is space for the lighter phase separating from the band and under, space for the heavier phase separating from the band. In this arrangement, there is actually no traditional settler unit at all. The extraction cells within the extraction step and in different steps of the extraction process are all similar.
A new method for reducing the size of solvent extraction process stages has been developed, where, as stated earlier, there are many extraction steps, especially in the solvent extraction of other metals than copper, and where there are many similar cells within the step. The guiding principle here is that the good separation capacity of the phases is important, especially in the first and last cell of the extraction step. However, within the step, a greater amount of residual droplets is allowed, wherein the size of the extraction step is reduced by decreasing the size of the cells within the step. This occurs by deacreasing the settler unit of the cells within the step and replacing it with a more compact solution, wherein the cross-section area of the settler unit of at least one cell or preferably all cells in the middle of the extraction step is at most half of the cross-section area of the first and last settler unit in the step. According to the present invention, the settler unit of the extraction cell comprises a pump unit and a settler unit located around the mixers. The profile of the settling unit is preferably rectangular. The essential features of the invention will become apparent in the attached patent claims.
In the method developed, the size of the extraction steps is reduced, which in practice means that the process fits into a smaller space than a conventional process arrangement. In addition, savings are made in investment costs and the need for solvent extraction chemicals is also reduced. An additional benefit worth mentioning is that the amount of capital tied up in the process will also decrease.